Hello,
What FroggEye meant is that you can check the final output code of the online form web part rendered on a live site using your browser (ctrl + u), in order to find out which CSS classes Kentico CMS assigns to each form element.
For example you can see the input field is rendered like this this:
<input name=".." type="text" maxlength="200" id="..." class="TextBoxField" />
When you have found the relevant classes, simply place your CSS to their definition in the site Stylesheet. You can also place additional classes to the
CMS Desk -> Tools -> Form -> Layout -> Source, when defining the custom layout of your form.
Regards,
Josef Dvorak